1. Field of the Invention
This invention pertains generally to detecting cracks and other anomalies in tubes such as steam tubes used in boiler applications. A method and apparatus are disclosed in which, in the preferred embodiment, a transverse magnetic-dipole source positioned inside a tube generates a continuous electromagnetic wave and electric field parallel to the axis of the tube. The wave reflects off the external wall of the tube and returns to a coaxial electric-dipole receiver. The transmitting and receiving antennas are designed and positioned to minimize crosstalk and at the same time distinguish cracks and other anomalies in the tube based on the return signal.
2. Description of the Related Art
Historically, and even to the present day, the major cause of boiler outages is the degradation of the steam tubes by which heat is extracted from the boiler. Although there is deposition and corrosion upon the internal surfaces of the tubes, the principal cause of failure is by the development of exterior cracks. Of course, when a boiler is otherwise down for repair, the exteriors of all apparently satisfactory tubes are inspected for cracking. Unfortunately, some cracks may not be discovered because the exterior surfaces are more or less covered by combustion or corrosion products. An inspection tool that could be passed through a boiler tube, and detect an unseen sidewall crack from within, would increase the intervals between boiler outage and provide substantial cost savings.
Other approaches for detecting cracks in metal and other structures have been described which utilize electromagnetic waves. Unlike the invention disclosed here, though, those techniques do not utilize radar backscatter. Crack and anomaly detectors that employ the use of eddy currents, for example, include the waste storage tank probe/scanner produced by ZETEC.TM. and the logging and survey tools marketed by TUBOSCOPE PIPELINE SERVICES.TM. (See: ZETEC.TM. INC., "Waste Storage Tank Probe/Scanner--Remote Field Testing (RFT) Probes", and "Introduction to Pipeline Services", "Survey Tool Operation" and "TruRes"). Eddy current and ultrasonic Lamb-wave techniques have been studied, and various combinations of pancake-coil arrays have been investigated and described in reports from Argonne National Laboratory. (See: D. R. Diercks, et al., "Overview of Steam Generator Tube Degradation and Integrity Issues", Argonne National Laboratory, Argonne, Ill.; S. Bakhtiari, et al., "Modeling of Eddy Current Probe Response for Steam Generator Tubes", Energy Technology Division, Argonne National Laboratory, Argonne, Ill.; and D. S. Kupperman, et al., "Characterization of Flaws in a Tube Bundle Mock-up for Reliability Studies", Argonne National Laboratory, Argonne, Ill.)
The concept of the present invention, by contrast, offers a technically different approach. Radar backscatter represents an improvement over existing technology because it offers the same or better resolution with respect to crack location and mapping than current methods while not requiring external components such as magnets (some eddy-current systems), immersion in water (some ultrasonic systems), or mechanical steering (pancake-coil systems). Likewise, the radar backscatter method of the present invention operates where a pulse-type radar system cannot. The dimensions of boiler tubes and oil/gas pipelines, for example, would require such a high-frequency system that the thickness of the metal wall would completely absorb the electromagnetic field before the field could reach the exterior.